A laser beam machine equipped with a laser oscillator is widely used as a machine tool for carrying out a machining operation, such as cutting or welding, on a workpiece made of metal or nonmetallic material. FIG. 1 is a perspective view schematically showing, by way of example, the arrangement of a laser beam machine which is generally indicated at 1. The laser beam machine 1 comprises a laser oscillator 2, a laser machine tool 3, and a numerical control device (NC device) 4. A laser beam output from the laser oscillator 2 is guided to the laser machine tool 3 through a shielding duct 5 serving as a light guide path and then reaches a machining head 6 of the laser machine tool 3.
The machining head 6 includes a converging lens for converging light which has been deflected downward by a deflecting mirror in the laser machine tool 3 after passing through the shielding duct 5. Vertical position of the machining head 6 can be controlled by a Z-axis mechanism, whereby the laser beam output from the machining head 6 is converged to a machining spot on a workpiece 7 placed on an X-Y table, the position of which is controlled by the numerical control device (NC device) 4. Usually, the laser beam converged to the machining spot is circularly polarized in order to ensure proper machining, and, therefore, it is desirable that when the laser beam is emitted from the laser oscillator 2, it has already been circularly polarized or linearly polarized at an angle of 45 degrees with respect to the horizontal plane. In the latter case, circularly polarized light can be obtained easily by arranging a phase delaying plate (1/4-wavelength plate) in the light path of the beam emitted from the laser oscillator 2, or by using a phase delaying mirror as the deflecting mirror in the laser machine tool 3.
The laser oscillator 2 is equipped with a gas excitation device including a discharge tube, and a laser resonator associated with a gas cooling unit etc. FIG. 2(a) through FIG. 2(c) schematically illustrate, by way of example, the arrangement of a conventionally employed laser resonator, along with additional elements such as a beam phase adjusting unit, wherein FIG. 2(a) is a plan view of parts of the laser resonator forming a resonant light path, as viewed from above a plane formed by the resonant light path, FIG. 2(b) is a front view of almost the entire laser resonator, as viewed from side with respect to the plane formed by the resonant light path, and FIG. 2(c) is a side view of the laser resonator, as viewed from the right-hand end face in FIG. 2(a).
Referring to these diagrams, a laser resonator, generally indicated at 8, comprises a machine frame 9, a gas excitation device 10, and a gas cooling unit 11. The machine frame 9 is composed of front and rear plates 12 and 13, each made of an aluminum plate, and four rods 14 connecting these plates so that the frame 9 may not be readily deformed by external force. Also, in order to minimize dimensional change of the machine frame 9 due to heat, each rod 14 is made of invar steel and is formed as a tubular member (resonator pipe), and cooling water is passed through the pipes 14 during operation of the laser resonator 8. That is, the machine frame 9 is designed so that thermal deformation thereof as a whole may be controlled to an extremely low level.
The gas excitation device 10 includes a discharge tube 15, a high-frequency power supply 16, an output mirror 17 arranged at a starting end (front plate 12) of the discharge tube 15, two bending mirrors 18, 18 arranged at an intermediate portion (rear plate 13) of the discharge tube 15, and a rear mirror 19 arranged at a terminal end (front plate 12) of the discharge tube 15. The gas excitation device 10 is fixed in position between the front and rear plates 12 and 13 via discharge tube holders 20. The bending mirrors 18, 18 are fitted in a reflector block 21 in such a manner that they face each other at an angle of 90 degrees therebetween. The high-frequency power supply 16 serves to produce an electric discharge between opposing electrodes arranged on the peripheral wall of the discharge tube 15, to thereby excite CO.sub.2 gas in the discharge tube 15. A laser beam emitted from the excited gas is amplified while traveling back and forth within the discharge tube between the output mirror 17 and the rear mirror 19, and part thereof is outputted frontward (to the left in FIG. 2(a)) from the output mirror 17 as a laser beam 22.
The gas cooling unit 11 is composed of a Roots blower 23, heat exchangers 24 and 25, respectively arranged on the intake and discharge sides of the blower 23, and a ventilating pipe 26. When the Roots blower 23 is in operation, air is circulated through the ventilating pipe 26 while the temperature thereof is adjusted by the heat exchangers 24 and 25, and flows round the surface of the discharge tube 15, though not illustrated, to cool the discharge tube 15.
Reference numeral 27 denotes a shutter mirror which is inserted across the light path when laser beam machining is to be temporarily suspended. When the shutter mirror 27 is in a shut state, the laser beam 22 is deflected from a main path for machining operation and is absorbed by a beam absorber 28.
Reference numeral 29 denotes a beam phase adjusting unit which includes therein a phase delay reflecting mirror 30 and a zero shift reflecting mirror 31. The beam phase adjusting unit 29 serves to convert a linearly polarized laser beam into a circularly polarized laser beam.
The foregoing is the typical arrangement of a conventional laser resonator. However, in order to ensure proper machining conditions with the laser resonator having an arrangement such as one described above, the overall size of the resonator must inevitably be increased. That is, in general, the laser beam output from the laser resonator 8 is converged to perform machining operation, and in order to ensure proper machining condition, there is a certain range for the distance (hereinafter referred to as "light path length") between the beam outlet of the laser resonator 8 and the machining spot, which will realize a good machining conditions. According to the result of an experiment in which cutting performance was tested using a beam of a CO.sub.2 gas laser, a desirable light path length was found to be a considerably large value of 3 m to 6 m (optimum light path length) as measured from the output mirror 17 of the laser resonator 8.
Thus, in the case where the conventional arrangement shown in FIG. 2 is employed as a part constituting the laser oscillator 2 disposed as shown in FIG. 1, suitable measures must be taken to ensure an optimum light path length for the laser beam 22 emitted from the output mirror 17. For example, the laser oscillator 2 must be separated from the laser machine 3 and be connected to the same by a long shielding duct 5, which, however, makes it difficult to obtain a compact laser beam machine. Also, it is difficult to obtain a sufficient light path length between the phase delay reflecting mirror 30 and the zero shift reflecting mirror 31 in the beam phase adjusting unit.
To shorten the light path length from the laser resonator to the machining spot, Unexamined Japanese Patent Publication (KOKAI) No. 5-235454 discloses a system in which additional reflecting mirrors are arranged in the laser resonator so as to turn back the laser beam emitted from the output mirror of the resonator, such that the laser beam is outputted from the laser oscillator after traveling for a predetermined light path length from the output mirror. This publication discloses a laser resonator arrangement wherein the additional reflecting mirrors are arranged in the vicinity of the output mirror such that the laser beam emitted from the output mirror is immediately reflected twice and then outputted in a direction opposite to the direction of emission from the output mirror. In the following, this improved type laser resonator is referred to as the "improved laser resonator."
FIG. 3 schematically illustrates an optical system employed in the improved laser resonator. In FIG. 3 and FIGS. 4 and 5 described later, like reference numerals are used to represent like or corresponding component parts in the conventional laser resonator shown in FIGS. 2(a) to 2(c).
Referring to FIG. 3, the laser resonator 8 includes two discharge tubes 15 arranged in parallel with each other, bending mirrors 18a and 18b facing each other at an angle of 90 degrees therebetween, an output mirror 17 arranged at the starting end of one discharge tube 15, and a rear mirror 19 arranged at the terminal end of the other discharge tube 15. Accordingly, the resonant light path is a generally U-shaped path formed by successively connecting the output mirror 17, one discharge tube 15, the bending mirrors 18b and 18a, the other discharge tube 15, and the rear mirror 19 one another. FIG. 3 illustrates two discharge tubes 15, but in practice, these discharge tubes 15 are connected to form a single discharge tube, which is filled with CO.sub.2 gas so that the internal space thereof may serve as a resonance space. The arrangement of the resonant light path is almost the same as that of the conventional type shown in FIG. 2.
The improved laser resonator is characterized in that two additional reflecting mirrors M.sub.1 and M.sub.2 are arranged so as to cause the laser beam to make a U-turn on a plane inclined at 45 degrees with respect to a plane (horizontal plane) formed by the resonant light path. Specifically, the first additional reflecting mirror M.sub.1 is arranged in the vicinity of the output mirror 17 such that the laser beam emitted from the output mirror 17 is bent by 90 degrees and then propagated in a direction which is within the plane inclined at 45 degrees with respect to the plane formed by the resonant light path. The second additional reflecting mirror M.sub.2 is arranged in such position and orientation that the laser beam reflected from the first additional reflecting mirror M.sub.1 is again bent by 90 degrees and then propagated in a direction opposite to, or different by 180 degrees from, the direction in which the laser beam is emitted from the output mirror 17. In order for the laser beam emitted from the laser; resonator to be circularly polarized light C, a phase delaying mirror is used for at least one of the two additional reflecting mirrors M.sub.1 and M.sub.2.
With this arrangement, when the laser beam 22 leaves the laser oscillator 2, it has already been propagated over at least a light path length corresponding to the longitudinal length of the laser resonator 8. Therefore, when connecting the laser oscillator 2 and the laser machine tool 3 shown in FIG. 1 to each other by a light path with the aforementioned optimum length, the length of the light path between the two (the length of the shielding duct 5) can be shortened correspondingly. Another arrangement which may be employed is one such that the laser beam reflected from the second additional reflecting mirror M.sub.2 is passed through a long focal-length lens before being emitted from the laser resonator 8 so that the angle of divergence of the laser beam 22 may be adjusted.
With the use of the improved arrangement of laser resonator, the dimension along the length of the laser resonator can be reduced, as described above. However, the following problems still remain unsolved:
(1) Since the additional reflecting mirrors M.sub.1 and M.sub.2 are set so as to be inclined at 45 degrees to the horizontal plane, the dimensional size of the laser resonator 8 along the height thereof becomes relatively large (indicated at h in FIG. 3), which is not desirable when making the oscillator compact.
(2) The phase delaying mirror used for at least one of the additional reflecting mirrors M.sub.1 and M.sub.2 to obtain circularly polarized light generally has short service life and is expensive. Therefore, this adversely affects the maintainability and the cost of the laser resonator 8.